On each square of an $n$x$n$ board sleeps a dragon. Two dragons are called neighbors if their squares have a side in common. Each turn, Minnie wakes up a dragon which has a living neighbor and Max directs it towards one of its living neighbors. The dragon than breathes fire on that neighbor and destroys it, and then goes back to sleep. Minnie's goal is to minimize the snoring of the dragons and leave as few living dragons as possible. Max is a member of PETD (People for the Ethical Treatment of Dragons), and he wants to save as many dragons as he can. How many dragons will stay alive at the end if 1. $n=4$? 2. $n=5$?
Problem
Source: Israel Autumn 2016 TST2/3
Tags: combinatorics, combinatorics solved
ThE-dArK-lOrD
01.10.2016 19:36
Any solution $?$
MellowMelon
01.10.2016 21:07
- For any subset $S$ of $k$ dragons such that every dragon is in $S$ or a neighbor of a dragon in $S$, Minnie can guarantee at most $k$ dragons survive, just by choosing the dragons in $S$ repeatedly until no others are left.
- For any subset $S$ of $k$ dragons such that no two dragons in $S$ are neighbors or have common neighbors, Max can guarantee at least $k$ dragons survive. He does so by never destroying a dragon in $S$ unless he has no other choice. In this way, either each dragon in $S$ survives, or it is destroyed by a dragon who is thus isolated from all other dragons and survives. These dragons don't overlap since no dragons in $S$ have common neighbors.
The following diagram shows the answer is 4 for #1. (same subset for both)
.O..
...O
O...
..O.
The following diagram shows the answer is at least 6 for #2.
O...O
..O..
.....
O...O
..O..
Label each dragon with $(x,y)$ for $1 \le x,y \le 5$. Minnie can guarantee 6 as follows.
- Have dragon $(3,3)$ destroy all its neighbors.
- Choose the dragon in $(1,1)$. WLOG, he destroys $(2,1)$.
- Have dragon $(1,2)$ destroy all its neighbors.
- Choose the dragon in $(1,5)$. The dragon not chosen is then made to destroy all its neighbors. This means one dragon out of $(1,4), (2,4), (1,5), (2,5)$ still lives.
- Minnie has dragons $(4,1), (5,3), (4,5)$ destroy all of their neighbors.
This leaves only 6 dragons alive.
Ismael10
13.11.2016 01:17
Isn't possible to be 9 for n=5 0.0.0 ..... 0.0.0 ..... 0.0.0